Field of the Invention
The present invention relates to machine tools such as lathes capable of performing, e.g., a threading process.
Description of Related Art
When machining is performed with a machine tool (in particular, machining of a workpiece with low rigidity or machining using a tool with low rigidity), strong vibrations known as chatter vibrations are often generated during machining. Such chatter vibrations cause problems such as a so-called chatter mark on the cutting surface, chipping of the tool, etc. As a solution to such problems, a technique has been devised which suppresses the chatter vibrations by changing the main spindle rotation speed during a threading cycle, as disclosed in, e.g., Japanese Patent Application Publication No. 2004-209558 (JP 2004-209558 A).
However, it is known from experiences that in the case of alternately performing cutting at a low rotation speed and cutting at a high rotation speed as in the above related art, cutting at the high rotation speed is less likely to cause chatter vibrations. Accordingly, if a specific tool pass that tends to cause chatter vibrations is known, it is desirable to perform cutting in this specific tool pass at the high rotation speed. In particular, in view of cutting surface accuracy, it is desirable to perform cutting in the last tool pass at the high rotation speed. However, in the case of changing the main spindle rotation speed in every plural passes, it is difficult to know whether the main spindle rotation speed of the first cutting at the start of machining should be the low rotation speed or the high rotation speed. Accordingly, cutting in the specific tool pass or the last tool pass may be performed at the low rotation speed, which may hinder effective suppression of chatter vibrations or degrade cutting surface accuracy. Prior to the present application, the applicant has devised a technique capable of suppressing an increase in cutting load. In the technique, the same run-out angle is used in all the passes when the main spindle rotation speed is changed on a pass basis during a threading cycle (Japanese Patent Application No. 2012-121107). The present invention has been developed in view of the technique.